Fetal monitors are devices for measurement visualization of one or more physiological parameters of unborn human beings. These monitors consist of multiple sensor elements for measuring uterine activity and one or more fetal heart rates. Basically two methods are used for electronic monitoring. The external or indirect method employs the use of external transducers placed on the maternal abdomen. Typically, ultrasound Doppler transducers are used in this category, where high frequency sound waves reflect mechanical action of the fetal heart. The internal or direct method uses a spiral electrode to receive a fetal electrocardiogram obtained from the presenting part of the unborn. This method can be used only when the presenting part is accessible and identifiable.
Monitors using ultrasound Doppler technologies normally accurately detect the fetal heart rate. However, electronic fetal monitors may unintentionally record maternal heart rate in the case of weak or absent fetal heart rate or if transducers are misplaced.
In addition to that, traces recorded with ultrasound Doppler can show the phenomenon of double counting. Double counting may occur if a maternal aortic wall movement during systole is nearly identical to the aortic wall movement during diastole. The envelope wave derived from the sensor signal then has identical shapes and the fetal monitor software cannot detect a difference between the two. Instead of counting a beat of the heart as one, two will be counted which can double the heart rate. Therewith, heart rate doubling occasionally occurs when measuring weak signals caused for example by aortic wall movements. Often the doubled maternal heart rate appears to have exaggerated variability and therefore may be interpreted as fetal heart rate.
Maternal heart rate patterns can mimic fetal heart rate patterns on such recordings. Misinterpreting a wrong heart rate trace may lead to unnecessary actions, unnecessary surgery, delayed delivery of a compromised fetus or even fetal death.
Because the fetal monitoring technology cannot detect the difference between a fetal and a maternal signal source when using fetal transducers, all fetal monitor manufacturers recommend producing a continuous maternal trace. Various techniques are known for this purpose. Firstly the pulse can be measured manually via stop watch. Secondly, the maternal heart rate may be obtained from a maternal pulse oximeter sensor placed on the maternal finger or ear. Thirdly, the application of the electrocardiogram device can be used to generate the maternal heart rate trace. Another possibility is to place a second ultrasound transducer over the maternal heart.
Therefore, most fetal monitors have built-in comparison algorithms for identifying identical fetal and maternal heart rates. The cross-channel verification feature helps detecting these trace coincidences. Question marks are automatically printed whenever two recorded heart rate traces show similarities over a certain amount of time.
US 2004/0243015 A1 discloses an apparatus for monitoring a fetal heart beat to extract one or more fetal electrocardiograms from a composite signal detected at the abdomen of a pregnant woman.
US 2005/0119583 A1 discloses a heart rate monitor apparatus and method.
US 2005/0267376 A1 discloses a maternal-fetal monitoring system for use during all stages of pregnancy.
US 2006/0229518 A1 discloses a method to analyze the fetus heart beat signal and detect a specific list of fetal heart rate arrhythmias in a non-intrusive, non-invasive and non-emitting way.
WO2005/110236 A1 discloses a beltless device for monitoring labor contractions via a fiber optic cable and fetal heart beats via an ultrasound sensor.
U.S. Pat. No. 6,178,343 discloses a pulse rate and heart rate coincidence detection unit for pulse oximetry.
State of the art methods to improve the fetal heart rate detection by additionally acquiring the maternal heart rate require either additional sensors or at least additional electrodes, which is a major disadvantage. Electrodes and sensors add additional cables, thus increasing patient's and caregiver's inconvenience. As a result, any method that adds additional sensors and/or electrodes is not well accepted.